Among photosensitive resin compositions, photolithographically processable ones are called photoresists and broadly used for semiconductors, MEMS and micromachine applications, and the like. In such applications, the photolithographic processing is accomplished by carrying out patterning exposure on a substrate and then selectively removing exposed regions or non-exposed regions by subsequent development with a developing solution. The photoresists are classified into positive and negative types. Exposed portions are dissolved in a developing solution in the positive type, while exposed portions become insoluble in the negative type. In electropackage applications and MEMS applications of advanced technologies, not only the capacity of forming uniform spin coating films but also a high aspect ratio, a vertical side wall shape in thick films, and high tight adhesion with substrates and the like are demanded. Here, the aspect ratio is an important property which is calculated using the ratio of a resist film thickness/a pattern line width and indicates the photolithographic performance.
Compositions having a bisphenol A novolac epoxy resin as a major component disclosed in Patent Literature 1 and Non Patent Literature 1 have a very high resolution and the use of the compositions enables formation of higher-aspect ratio photosensitized images and photosensitive resin cured products. However, the resin cured products obtained using the compositions pose such problems that substrates are largely warped after photolithographic processing using silicon wafers or the like as the substrates thereby damaging fabricated devices and decreasing the yields due to their remarkably high residual stress values. Further, due to the high residual stress, cracks (crazings) are generated at the time of development in some cases and exfoliation is easily generated between the substrates and the resin cured products in some cases.
Patent Literature 2 discloses that a resin composition containing a bisphenol epoxy resin, an alicyclic epoxy resin and a cationic photopolymerization initiator is excellent in the adhesiveness with a Ni/Si wafer. Patent Literature 3 discloses that a photosensitive resin composition containing an epoxy resin having a specific epoxy equivalent and a specific softening point, a phenolic curing agent having a specific hydroxyl equivalent, and a photoacid generator is excellent in the adhesiveness with a silicon wafer. Further, Patent Literature 4 discloses that a photosensitive resin composition containing a bisphenol epoxy resin, a phenol novolac epoxy resin having a specific structure, and a cationic photopolymerization initiator is excellent in the adhesiveness with a silicon wafer. As a result of studies by the present inventors, however, it has been found that the resin compositions in these Patent Literatures were insufficient in the adhesiveness with metals (substrates) such as Pt and Ta after a test for moisture and heat. Further, the residual stress of the cured products was very high and warping of the substrates was remarkable.
On the other hand, in recent years, in MEMS applications utilizing photolithographic processing, development has been conducted for manufacture of frequency filter elements such as surface acoustic wave filters mounted on communication terminals. As a typical one, Patent Literature 5 discloses a technique in which a frequency filter element can be manufactured in a small size and at a low cost by forming an interdigitated electrode in which a plurality of metal films are layered on a piezoelectric substrate of lithium tantalate (LT) or the like and using an organic resin photolithographically processable as an insulating film on the electrode. However, there have been such defects that since the polyimide necessitates a high-temperature curing condition, it causes damage to devices; and the epoxy resin can be cured at a low temperature, but to due to a high residual stress of the cured product, the warping of the substrate, the cracks of the film, and the degradation in the tight adhesion on the Pt film and the LT substrate after the moisture and heat treatment tend to occur.